Washington — Louisiana Sen. Mary Landrieu – one of the vulnerable Democrats up for reelection this fall – is deploying a special campaign weapon: her new post as chairwoman of the Senate Committee on Energy and Natural Resources.

She touts it in campaign ads and from the halls of the Senate. As a senior lawmaker from a big energy state, that insider influence ought to help her keep her seat and perhaps even help Democrats retain control of the Senate. Or so the thinking goes.

A few weeks ago, Senator Landrieu promised to use her gaveling arm to force approval of the controversial Keystone XL pipeline. This, after President Obama had again announced a delay in his decision on whether to approve the pipeline that would bring oil from Canada’s tar sands down to the Gulf of Mexico. Landrieu spoke out forcefully about the position she gained this year: “I plan to use my power as chair of the Senate Energy Committee to take decisive action to get this pipeline permit approved.”

But in the first significant test of that power this week, she couldn’t deliver. Her bipartisan legislation to approve Keystone, co-sponsored by Sen. John Hoeven (R) of North Dakota, stalled and then died in a fight over amendments on a related energy-efficiency bill.

Now, her election is developing as a test of how valuable something like a committee chairmanship can be for an incumbent. Once a sought-after prize, chairmanships appear to be increasingly devalued by how Congress has evolved and voters' cynical views of Washington power brokers.

For her part, Landrieu acts as if it’s quite valuable. Her first TV ad of the season, which portrays her fighting the Obama administration on oil and gas, bragged about it. Another ad, released at the end of April, featured a Louisiana Republican shipbuilder, Boysie Bollinger, saying that while he may not always agree with her, the state “can’t afford to lose” Landrieu. As the chair of the energy committee, the three-term senator has “the most powerful position a person can have for Louisiana,” he says as he strolls past his behemoths under construction.

He has a point, says Kirby Goidel, a political scientist at Louisiana State University in Baton Rouge. After Katrina, Landrieu became “a pretty strong voice” for Louisiana. She’s encumbered now with the unpopular Affordable Care Act and other issues, but she can use her committee to help differentiate herself from Mr. Obama, as she did on Keystone. The prospect of losing one of the state’s most powerful members in Washington “is a pretty big appeal for Senator Landrieu and many in the business community who might otherwise tend to support a Republican,” says Professor Goidel.

But a candidate invites criticism when she promises something and doesn’t deliver, he adds. Indeed, when Keystone imploded with a procedural vote on Monday, Landrieu’s main Republican competitor, Rep. Bill Cassidy from the US House, issued a press release titled: “Landrieu’s ‘Influence’ Fades on Keystone.” Landrieu and Mr. Cassidy are running a very close race, according to polls.

“When you raise the issue that you’re influential, you’ve got to prove it with results,” warns Ross Baker, a congressional expert at Rutgers University and author of a forthcoming book, “Is Bipartisanship Dead?”

One problem for Landrieu, he says, is that committees – and committee chairs – aren’t as powerful as they used to be. Much committee work has been commandeered by the leadership in the Senate and the House. And leadership is consumed with positioning for the next election, he says.

In this case, Senate majority leader Harry Reid (D) of Nevada cared more about protecting his base from unpopular GOP energy amendments than he did about Landrieu, Mr. Baker says. “It’s the larger party message that was involved. He’s got a constituency that’s much broader than Mary Landrieu – as much as he wants to have her elected.”

The bottom line, of course, is what voters think about insider power. According to an April poll of likely Louisiana voters by Southern Media & Opinion Research Inc., the answer is not much. When told that Landrieu has been a senator since 1996 and that she recently became chair of the Senate Energy Committee, 59 percent of voters thought that electing someone new was more important than keeping her in office.